A biologic process that rapidly expands vascular networks is intussusceptive (nonsprouting) angiogenesis. Vascular expansion occurs by the active subdivision of one vessel into two lumens. The earliest stage of this process is characterized by the formation of tissue islands or intraluminal pillars. We have found these intravascular pillars in the skin (dermatitis), colon (colitis) and lung (post-pneumonectomy). Growth or extension of the intraluminal pillars down the axis of the vessel leads to vessel duplication. The rate of pillar extension and vascular division appears to far exceed the proliferative capacity of resident endothelial cells. This project addresses the following question How is the process of adult intussusceptive angiogenesis capable of such rapid blood vessel expansion; further, how can this process be harnessed for therapeutic angiogenesis? To answer this question, we have developed a parabiotic mouse model of post-pneumonectomy intussusceptive angiogenesis. In the parabiotic model, wild-type and green fluorescence protein (GFP) mice are surgically paired to establish complete cross-circulation. After establishing a shared circulation, a pneumonectomy is performed in the wild-type mouse. Morphometry demonstrates that the remaining lung develops 1-3 kilometers of new alveolar capillaries within 2 weeks of pneumonectomy. Corrosion casting and 3-dimensional scanning electron microscopy shows that this blood vessel growth is largely produced by intense intussusceptive angiogenesis. An important clue to understanding the process of intussusceptive angiogenesis is the observation that post-pneumonectomy vessel growth is associated with GFP+ endothelial cells. Since the intussusceptive angiogenesis occurs in the wild-type mouse, GFP+ cells in the vascular lining must be derived from the peripheral blood circulation. These GFP+ endothelial progenitor cells (EPC) not only demonstrate the importance of EPC in intussusceptive angiogenesis, but they provide an explanation for the rapid rate of vascular expansion. Adding further support for the importance of EPC, we have demonstrated that, in studies of parabiotic colitis, blood-borne EPC localize near intussusceptive pillars. To test our hypothesis that intussusceptive pillars localize circulating EPC, we will use the parabiotic model to track endothelial progenitor cells (EPC) from their mobilization in the bone marrow to their migration into the lung during parabiotic post-pneumonectomy intussusceptive angiogenesis (Specific Aim #1). Advanced imaging will determine the effect of intussusceptive pillars on EPC localization, vascular integration and total angiogenesis (Specific Aim #2). Finally, the intravascular interaction of EPC and pillars during intussusceptive angiogenesis will be modeled in vitro and in silico (Specific Aim #3). The goal of this project is the functional regulation of adult intussusceptive angiogenesis; that is, the rapid expansion of microvascular networks for application in both regenerative medicine and tissue engineering.